This invention relates to a method of producing containers, more particularly producing containers from thermoplastic sheet material for packaging purposes. Containers of this kind are especially used for packaging food.
For producing containers of thermoplastic sheet material a molding die is used which is advanced towards and against a heated sheet material which is clamped at its periphery to be in a generally flat state. Conventional molding dies have a raised or projecting peripheral portion and a recessed central portion so that the containers formed thereby will have a recessed bottom. On its bottom side directed towards the sheet material, the molding die has preferably rounded edges, just as on the edge portion of the side walls adjacent the bottom. For shaping the preheated sheet material or foil, the molding die is substantially normally moved towards the sheet material or foil towards a container the shape of which corresponds to the shape of the molding die. Those portions of the sheet material or foil which are first to contact the molding die are cooled by transmission of heat to the molding die the temperature of which is below the temperature of the plastic state of the thermoplastic sheet material. Thus, those portions not contacting the surface of the molding die at the time a first contact is made will be stretched more than other portions. The stretching effect will be maximum at the edges, particularly at any corners to be formed in the container. The edge and corner portions of the containers are thus formed with reduced thickness, and this is undesirable in view of producing stable containers.